Hi,
It looks like MGRS coordinates. No telling what GCS the coordinates are based on, but the values are only to the nearest kilometer so it doesn't matter.
28RCS1931
UTM zone 28N, R = latitude band between 24N and 32N, etc.
latitude = 28.29262
longitude = -16.84571
The lat/lon values are not that precise, by the way. Kilometer accuracy is around 2-3 decimal places when working in decimal degrees.
Melita